Tens of thousands of Egyptians returned to Tahrir Square in Cairo on Friday in a show of national unity against sectarian tension, and to demonstrate their solidarity with the Palestinian people.

The largest rally to be held in the Egyptian capital in recent weeks took place as Suzanne Mubarak, wife of ousted president Hosni Mubarak, was detained by investigators for 15 days on suspicion of illegally acquiring wealth.

Cheers erupted in the square as news broke of Mrs Mubarak's incarceration. The 70-year-old former first lady now joins her husband, two sons and more than 20 other ministers and business figures from the Mubarak regime on the list of those being investigated for crimes against the state.

Last week former interior minister Habib Al-Adly was sentenced to 12 years in prison for financial fraud. He also stands accused of having ordered the killing of peaceful protesters, a charge that can carry the death penalty.

In a sign of how vibrant and fragmented Egypt's political landscape has become since the toppling of Mubarak in February, protesters came together on Friday to support a multitude of causes from local anti-corruption campaigns to unity with Arab uprisings elsewhere in the region.

Following a week of sectarian violence in Cairo in which at least 15 people were killed in clashes at a church in the poor neighbourhood of Imbaba, many demonstrators held aloft placards depicting the Christian cross and Muslim crescent, and chanted: "We are all Egyptians."

On Wednesday the UN secretary general, Ban Ki-moon, said he was disturbed by the religious fighting, warning that it could threaten progress towards a "more free, just and harmonious Egypt". Egypt's Coptic Christian community comprises about 10% of the population, and claims it has long been the target of severe discrimination in the Muslim-majority country.

"I think there are two themes playing out in Tahrir today," said Hossam Bahgat, a human rights campaigner and director of the Egyptian Initiative for Personal Rights. "One is the search for a lost moment: in every speech on the stage and on every leaflet handed out there are words to the effect of 'do you remember [the anti-Mubarak protests in] January and February when we stood here together? What happened to that moment?'

"The second is the realisation that even if we succeed in re-creating that moment, we need to build a bridge between on the one hand that sentiment of unity which once overwhelmed Tahrir, and on the other the poorer rural and urban neighbourhoods, where it takes only the slightest thing or most absurd rumour to unleash large-scale communal violence."

Egypt's interim government has arrested almost 200 suspects in the aftermath of the Imbaba violence, charging 23 of them with terrorism and premeditated murder. The clashes began when Salafist Muslims marched on a church where they claimed a female Copt who had converted to Islam was being held hostage, prompting a street battle in which shots were fired and Molotov cocktails thrown.

An inquiry blamed hardline Salafists for the attacks, but hinted that elements of the old regime seeking to sow discord may also have played a part in the violence.

"The policies of the Mubarak regime did actively contribute to a rise in sectarian tension," argued Bahgat. "But at the same time we need to acknowledge that whoever has been involved in incidents of sectarian violence over the past few weeks are the same people that have been engaging in such violence for years. They are not hired thugs, they are not organised Islamic entities, and they are not elements of the previous regime – they are us. And until we recognise that they are us, the solution will remain elusive."

Protesters in Tahrir also hoisted aloft Palestinian flags to demonstrate their support for a recent Fatah-Hamas reconciliation agreement signed in Cairo, just two days before a planned march to the Gaza border to mark the 63rd anniversary of the "nakba" – an Arabic word meaning "catastrophe" which is used to describe the founding of Israel.

As part of a region-wide set of demonstrations in favour of Palestinian refugees being allowed to return home, protest leaders in Egypt have called for a "third intifada" to be launched on Sunday, with a mass march from Cairo to the Rafah border post.

Hamas leader Khaled Meshaal has discouraged the event, arguing that in its present state of transition Egypt cannot carry the "burden" of a "direct clash with the Zionist entity".

The Egyptian government has put all entry points to the Sinai Peninsula on high alert in an effort to stop the march. "Our chants against sectarian tension and in support of the Palestinians are not side-issues," said Ibrahim Houdaiby, 27, a political activist who was formerly a member of the Muslim Brotherhood.

"Without solidarity between Christians and Muslims, without justice for Palestine, our revolution will die. Today you're witnessing an attempt to keep it alive."